docs(build): use react-static for prerender

[layershifter]

Fixes #2826.

[layershifter]

It's finally ready! ✌️

Testing

Develop build

yarn start

Staging build

1. Update docs.js

-build().then(cb)
+build({ staging: true }).then(cb)

2. yarn build
3. yarn serve docs/dist

[layershifter]

/cc @levithomason

[levithomason]

👍 pushed a fix to .gitignore for the /tmp directory
👍 The development workflow with yarn start works OK

⛔️ Changing docs.js to build({ staging: true }) and running yarn build produces this error:

[12:43:17] Starting 'build:docs:static:build'...

ReferenceError: regeneratorRuntime is not defined
    at Object.<anonymous> (static.routes.js:28:1)
    at Module._compile (module.js:652:30)
    at Module._compile (node_modules/pirates/lib/index.js:91:24)
    at Module._extensions..js (module.js:663:10)
    at Object.newLoader [as .js] (node_modules/pirates/lib/index.js:96:7)
    at Module.load (module.js:565:32)
    at tryModuleLoad (module.js:505:12)
    at Function.Module._load (module.js:497:3)
    at Module.require (module.js:596:17)
    at require (internal/module.js:11:18)

(node:31600) UnhandledPromiseRejectionWarning:
    TypeError: Cannot read property 'DIST' of undefined
    at _callee$ (node_modules/react-static/lib/commands/build.js:73:58)
    at tryCatch (node_modules/regenerator-runtime/runtime.js:65:40)
    at Generator.invoke [as _invoke] (node_modules/regenerator-runtime/runtime.js:299:22)
    at Generator.prototype.(anonymous function) [as next]
        (node_modules/regenerator-runtime/runtime.js:117:21)
    at step (node_modules/react-static/lib/commands/build.js:31:191)
    at node_modules/react-static/lib/commands/build.js:31:437
    at new Promise (<anonymous>)
    at node_modules/react-static/lib/commands/build.js:31:99
    at build (node_modules/react-static/lib/commands/build.js:165:17)
    at gulp/tasks/docs.js:60:3

(node:31600) UnhandledPromiseRejectionWarning: Unhandled promise rejection.
             This error originated either by throwing inside of an async function
             without a catch block, or by rejecting a promise which was not handled
             with .catch(). (rejection id: 2)

(node:31600) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated.
             In the future, promise rejections that are not handled will terminate the
             Node.js process with a non-zero exit code.

[12:43:17] The following tasks did not complete:
           build, <parallel>, build:docs, build:docs:static:build

[12:43:17] Did you forget to signal async completion?

I debugged this for a bit but could not get anywhere.

[layershifter]

@levithomason React-Static forces the usage regeneratorRuntime. However, I think we don't need it for our builds. So this configured with NODE_ENV, ref.

It seems that I've forgotten to update package.json, because I've tested with yarn prerelease.
Pushed a small commit with an update, my fault 😼

[layershifter]

@levithomason resolved merge conflicts 👍